Attention Deficit Hyperactivity Disorder (ADHD) is a genetically influenced brain disorder with a prevalence of 3-5% in school-age children. Affected sibling pairs and densely affected multi-generation Hispanic families have been recruited. A genome-wide search for loci linked to ADHD has been completed. Participating individuals undergo a battery of psychological tests and have blood drawn for the linkage analysis and positional cloning studies that are being used to search for genes associated with ADHD. To date, we have complete information on 15 large, multi-generation Hispanic families. Simulation studies on 27 informative pedigrees indicate excellent power to detect linkage (Arcos-Burgos et al., 2002). The next phase of our research on ADHD involves US families with ADHD.